An absorption-type heat pump of the type which uses water and an aqueous lithium bromide solution is disclosed in Japanese Published Unexamined Patent Application No. 55-63364, for example. This system consists chiefly of an evaporator, an absorber, a regenerator, and a condenser. In the evaporator, water (refrigerant) is evaporated via a plurality of heat transfer tubes through which a first heating fluid passes. The water vapor from the evaporator is absorbed into an aqueous solution of lithium bromide (absorbent) in the absorber, so that a heat carrier fluid passed through the absorber is heated by the heat of absorption generated in the course of the absorption. In the regenerator, the diluted absorbent received from the absorber through a heat exchanger is heated with a second heating fluid so that the absorbed water is evaporated from the absorbent. The concentrated absorbent resulting from the regeneration process is returned through the aforementioned heat exchanger to the absorber, in which it is reused for the water vapor absorption. In the condenser, the water vapor introduced thereinto from the regenerator is cooled to condense by a plurality of heat transfer tubes through which a cooling fluid is caused to flow.
In the above prior-art heat pump, the evaporation of water in the evaporator is carried out indirectly through the intermediary of the heat transfer tubes through which the first heating fluid is caused to flow. Therefore in order to improve the rate of heat exchange between the water and the heating fluid, it is necessary to increase the heat transfer tubes in number so as to increase the area of heat conduction, consequently leading to a cost disadvantage. Another problem with such heat pump is that even if the heat transfer tubes are increased in number, it is yet inevitable that the outlet temperature of the heating fluid is higher than the temperature of the water vapor produced, the full utilization of the heat of the heating fluid being impossible despite a utilizable temperature difference still remaining between the heating fluid and the water vapor.